A novel equiatomic CoCrNiTiV high entropy alloy was fabricated by a vacuum arc-melting, and investigated from the view of phase component, microstructure, mechanical properties. The results experimentally displayed that a typical single phase BCC solid solution was acquired in the as-cast CoCrNiTiV alloy, while another BCC phase together with an FCC phase and a (Ni, Co)₃Ti phase emerged through annealing. In mechanics performance, the ultimate strength significantly increases from 1729 ± 15 MPa of the as-cast alloy to 2820 ± 15 MPa of the annealed alloy, while a relatively high hardness of 825.8 ± 16.3 HV and 680 ± 23.9 HV was obtained. It was suggested that the majority BCC phase and other precipitation both take the responsibility for the good synergy in strength and hardness.

Due to their excellent mechanical and physical properties, such as high strength, high hardness, good ductility, and superior high-temperature oxidation resistance etc.,^1–6) high entropy alloys (HEAs), proposed in 2004 by Yeh et al.⁷⁾ and Cantor et al.⁸⁾ independently, have drawn much attentions in the materials science. Initially, they are loosely defined as the solid solution alloys which are composed of ≥5 principal elements with an atomic fraction range of 5–35 at%. However, only insignificant multi-component alloys to date was experimentally confirmed possessing a typical single phase solid solution structure. Moreover, the exploitation of the alloys with the single phase solid solution structure mainly depended on the time-consuming trial and error. Thus, with an aim to predict the phase formation in the HEAs, many semi-empirical parameters,^9–11) such as configuration entropy, atomic size difference, electronegativity difference, were successively proposed. Unfortunately, their validity is always constrained in some specific realm of HEAs. This is to say that more investigations about the single phase solid solution HEAs basing on experiments are necessary and urgent.

To explore the microstructure and phase component of novel HEAs, Cai et al.⁹⁾ first studied an equimolar CoCrNiTiV 3d transition metal (3d TM) HEA prepared by a vacuum hot-pressing sintering. They experimentally showed that the alloy consists of a body centered cubic (BCC) phase, a face centered cubic (FCC) phase, an (Ni, Co)₃Ti phase, TiO and Ni₂V₃ intermetallic compounds. As is well-known, the microstructure and phase component of an alloy are actually sensitive to its manufacture process. For instance, according to the investigation by Dias et al.,¹⁰⁾ the phase component of a CuCrFeTiV HEA is a typical single-phase BCC in as-milled condition, while a typical multi-phase structure (BCC+FCC) of the alloy was displayed in as-sintered condition. Thus, different microstructure of the CoCrNiTiV 3d TM HEAs would be initiatively expected under different prepared routes. In fact, if only considering the atomic size, the similar atomic radii among Ni, Cr and Co are beneficial to the formation of solid-solution rather than the mentioned multi-phase structure. On the other hand, the crystal structure of Ni, Cr and Co at room temperature (RT) is FCC, BCC and HCP, respectively. Hence, to uncover the microstructure of the CoCrNiTiV 3d TM HEAs, another manufactured technology is necessary to be performed.

Furthermore, Ti and V with larger atomic radii relative to Ni, Cr and Co, were usually added into HEAs due to its advantageous role on the enhancement of mechanical properties.⁹⁾ For instance, Ti was added into CoCrFeNi remarkably improving the ultimate strength from 413 to 2020 MPa in CoCrFeNiTi alloy.^12,13) The yield strength of the CoCrCuFeNiTi_x (x = 0, 0.5, 0.8 and 1.0) alloys increases from 230 MPa to 1272 MPa as the increasing Ti content, among which the CoCrCuFeNiTi_0.5 alloy exhibits ultimate strength of up to 1650 MPa together with extensive work hardening and large plastic strain limit of 22%.¹⁴⁾ Similarly, a continuous strengthening in CoCrFeMnNiV_x (x = 0, 0.25, 0.5, 0.75, 1) alloys was obtained with an increase of V content due to the increasing volume fraction of strengthening sigma-phase.¹⁵⁾

Thus, in this work, the HEA, NiCrCoTiV, with Ti and V incorporation, was fabricated by the most widely applied vacuum arc melting. Subsequently, its phase component, microstructure and mechanical behavior were comprehensively investigated. Comparatively, the microstructure of the present alloy is distinctly different with that in the as-sintered condition. Moreover, a relatively promising strength was obtained through annealing treatment.

CoCrNiTiV ingots were obtained by mixing the principal elements with purity higher than 99.9 at% under vacuum arc melting. The arc melting was conducted on a water-cooled copper crucible under 300 A electric current and a titanium-getter argon atmosphere. Four times re-melting and flipping each time was carried to ensure the ingots’ compositional homogeneity, and then the melt was poured into the water cooling Cu mould to form a cylinder samples (Φ4 mm × 6 mm). The actual alloy composition for the cylindrical samples was measured by energy dispersive spectrometry (EDS), as recorded in Table 1. To relieve the effect of rapid solidification on elemental distribution and microstructure, the as-cast cylindrical sample was annealed under argon atmosphere condition at 1073 K for 20 h.

Table 1 Nominal and actual compositions (at%) of the studied CoCrNiTiV alloy.

Phase components in the as-cast and annealed alloys were identified using a PANalytical X’Pert Powder diffractometer operating at 40 kV and 40 mA with Cu Kα radiation. The incident wavelength λ was 0.15418 nm, and the diffraction angle 2θ was tested from 30° to 100°. The microstructure of polished sample was analyzed using a field emission scanning electron microscope (FESEM, Zeiss sigma 500). To ensure a reliable microhardness, at least ten points for the hardness were tested on the polished cross-section surfaces from edge to center, using a Vickers hardness tester (HVS-1000B) under a load of 50 g for 10 s. The cylindrical specimens with ∼Φ3.7 mm × 5.6 mm were compressed parallel to their axis at RT by using a computer-controlled Instron (Instron, Norwood, MA) mechanical testing machine. To reduce friction, a thin Teflon foil was sandwiched between the compression faces and silicon carbide dies. The initial strain rate was set as 10⁻³ s⁻¹.

X-ray diffraction (XRD) patterns of the as-cast and annealed CoCrNiTiV alloys, together with the as-sintered one for comparison extracted from Ref. 9), were shown in Fig. 1. It is seen that the as-sintered CoCrNiTiV alloy was readily identified as a typical multi-phases structure, containing a BCC phase, an FCC phase, a (Co, Ni)₃Ti phase, a Ni₂V₃ intermetallic and a TiO compounds (Fig. 1(c)). Thereinto, TiO phase is attributed to the incorporation of O element during the preparation process. However, even if without consideration about the TiO phase, the Ni₂V₃ intermetallic, that is commonly formed at low temperature of ∼436 K, is actually non-existent in the as-cast and even as-annealed CoCrNiTiV alloy, which might originate from the sluggish diffusion effect of high entropy alloys. Likewise, the CuCrFeTiV HEA prepared by mechanical mixing last 6 h belongs to a typical BCC phase, while an extra FCC phase emerges in the alloy under spark-plasma sintering condition, which was identified as Cu-rich phase segregated from the BCC matrix caused by the weaker binding force between Cu and the other elements.¹⁰⁾ The difference in phase components for the two reported CuCrFeTiV HEAs was attributed to the effect of the distinct temperature and pressure, which is up to the preparation methods, on the atomic diffusion and binding forces. Therefore, it was suggested that the phase components of an alloy were closely related with the preparation process. Moreover, according to the XRD patterns, the as-cast alloy attractively possesses a single-phase BCC solid solution structure. As we know, the valence electron concentration ($\textit{VEC} = \sum\nolimits_{i = 1}^{n}c_{i} (\textit{VEC})_{i}$, c_i is the atomic mole fraction of i element)^16–18) is an effective parameter to predict the phase component. To be specific, a single FCC phase will be stable in alloys when VEC is not less than 8.0; a mixed BCC + FCC phases will form when VEC between 6.87 and 8.0, and a single BCC phase exists when VEC is no more than 6.87.^19,20) Thus, the low VEC of CoCrNiTiV (VEC: 6.8) indicates the studied alloy tend to form a single-phase BCC solid solution, which is consistent with the phase component identified by the XRD pattern. Combined with the XRD pattern and the Bragg equation, the lattice parameter of the BCC phase was determined to be a = 294.3 pm.

Fig. 1

The XRD patterns of the as-cast and annealed CoCrNiTiV alloys of this work (a) and (b) and the as-sintered one (c) extracted from the Ref. 9).

After annealing, the number of the XRD peaks obviously increased, which were readily identified belonging to two BCC (BCC1+BCC2), an FCC and a (Co, Ni)₃Ti phases. The typical multi-phase component might originate the variation of crystal structure of the principal element, and implied that the validity of VEC parameter is limited in this alloy. Besides, from the XRD, the insignificant peak intensity corresponding to the (Co, Ni)₃Ti phase, which has a hexagonal close-packed (HCP) (D0₂₄) structure, implied its content is marginal. Moreover, according to the JCPDS cards, the BCC1 and BCC2 should correspond to VCr and CoTi, respectively. Thus, it can be deduced that the FCC should be a Ni-rich phase considering the equal atomic ratio of the studied alloy. Besides, although the precipitation of the extra FCC and (Co, Ni)₃Ti phases, the BCC1+BCC2 phases are still the majority of phases (87.8 vol%), according to the calculated equation, $\text{W}_{\text{pi}} = (\text{p}/(\sum \text{p}_{\text{i}} )) \times 100\% $ (where W_pi is the volume fraction of the ith phase, p is the peak intensity of a given phase in the XRD pattern, i is the number of the phase, and $\sum \text{p}_{\text{i}} $ is the total peak intensity of all phases in the XRD pattern, respectively). Meanwhile, the lattice parameter of the BCC1 and BCC2 phases in the annealed condition are determined to be a = 296.7 pm and 295.8 pm, which are close to that of the as-cast alloys. The lattice parameter of the FCC and (Co, Ni)₃Ti phase are determined to be a = 359.6 pm and a = 512.5 pm, c = 837.4 pm.

As well known, arc-melting is the most widely applied method for preparing HEAs, which is usually conducted by arc melting the principal elements in a cold copper hearth and then solidifying them therein. During this process, the nonequilibrium phase or microstructure will be generated accompanied with the potential segregation and inhomogeneous microstructure. Thus, the above transformation of the XRD pattern from the as-cast to the annealed alloy should be attributed to the driving force derived from the Gibbs free energy difference between the nonequilibrium and equilibrium phases. This convinced that the post-process annealing is commonly necessary to establish the number and type of phases at equilibrium.²¹⁾ However, the standard thermal treatment process is still lacking for the estimation of achieving equilibrium, which is call for a comprehensively investigation in spite of out of the present research.

The microstructures of the as-cast and annealed CoCrNiTiV alloys are shown in Fig. 2(a)–(b) and (c)–(d), respectively. The as-cast sample was displayed a typical dendrite structure with an average primary arm size of around 12 µm, as shown in Fig. 2(a) and (b). Apparently, the dendritic regions (grey part marked as D) randomly embedded into the overall scenery. The rest of regions further divided into two distinct features: light white regions termed as interdendritic (marked as ID) and white part with a net-work shape. Considering no features depending on XRD patterns to distinct grey, light white and white regions, the EDS analysis was thus conducted to profile the chemical compositions of the mentioned regions and the relative results have been listed in Table 2. Hereby, we can figure it out that the D regions are mainly enriched with Cr and V but relatively depleted of Co, Ti and Ni, while the ID regions is mainly enriched with Co, Ti and Ni but relatively depleted of Cr and V. Besides, the compositional gradient in the white regions (Ni-rich regions) from high concentration to low one is Ni, Ti, Co, V and Cr. To further verify the overall distribution of each element, the EDS mappings are also carried out in the present work as shown in Fig. 3 and 4 for the as-cast and annealed alloys. According to Fig. 3, three distinct regions, belonging to CrV-rich, CoTi-rich and Ni-rich regions, strongly support the compositional distribution analyzed above. In fact, the compositional segregation in an alloy with a single phase solid solution structure is common in as-cast HEAs, such as the single BCC AlCoCrFeNi, AlCoCrFeNiNb_0.1²²⁾ and single FCC CoCrFeNiTi alloys.¹³⁾ Besides, the overall average compositions are very similar to the nominal compositions as shown in Table 1, indicating the melting reliability the currently studied alloy.

Fig. 2

SEM images of as-cast (a), (b) and annealed (c), (d) CoCrNiTiV HEAs.

Table 2 Quantitative chemical analysis of CoCrNiTiV samples in as cast and annealed states (at%).

Fig. 3

EDS mapping of Co, Cr, Ni, Ti and V of the as-cast CoCrNiTiV HEA.

Fig. 4

EDS mapping of Co, Cr, Ni, Ti and V of the annealed CoCrNiTiV HEA.

After annealing, the morphology, as shown in Fig. 2(c) and (d), displays some significant changes compared with the original as-cast state. Notably, different with the as-cast condition, the bright regions seem to be constituted by discontinuous precipitated phase at the high magnification image of the annealed alloy (Fig. 2(d)). Similarly, the EDS analysis was also carried out to profile the chemical compositions for different regions, which are in very good agreement with the EDS mapping (Fig. 4). Hereby, the dendritic regions were determined as CrV-rich BCC1 phase, and the interdendritic regions were CoTi-rich BCC2 phase. In the bright regions, the average concentration of Ni is up to 35.7 at%. Interestingly, the atomic ratio of (Ni + Co):Ti in there is close to 3:1, due to which we suggested that (Ni, Co)₃Ti identified by XRD should relate to the bright regions, which is consistent with the (Ni, Co)₃Ti phase of as-sintered CoCrNiTiV alloy either in composition or morphology.^23,24) The Co incorporation in the (Ni, Co)₃Ti phase might originate from a part of Ni substituted by Co in Ni₃Ti-like structural phase, since Ni and Co elements have similar atomic sizes and chemical properties. Besides, the elemental concentrations in the dendritic and interdendritic regions are similar to those identified in the as-cast alloy, thus deducing that (Ni, Co)₃Ti bright phase likely precipitates from part of the Ni-rich FCC regions via annealing.

Figure 5 shows the engineering stress (σ) vs. strain (ε) curves of the as-cast and annealed CoCrNiTiV alloys obtained during compression testing at RT. Their ultimate compressive strength σ_b, plastic strain ε_p and Vickers hardness HV_0.05 for both states are given in Table 3. Accordingly, it is suggested that the alloys either in the as-cast (AC) or annealed (H) conditions possess a relatively high ultimate strength (AC: 1729 ± 15 MPa and H: 2820 ± 15 MPa), but an extremely limited and even negligible ductility. Besides, the hardness dramatically decreases from 825.8 ± 16.3 HV of the as-cast alloy to 680 ± 23.9 HV of the annealed alloy, which are highest than the previously reported 5-principal elemental CoCrNi-based 3d TM HEAs, as seen in Fig. 6.^22,25–28)

Fig. 5

Engineering stress-strain curves for as-cast (AC) and annealed (H) CoCrNiTiV alloys.

Table 3 Ultimate compressive strength σ_b, plastic strain ε_p and hardness HV_0.05 of the as-cast and annealed CoCrNiTiV alloys.

Fig. 6

Hardness of the as-cast and annealed CoCrNiTiV alloys (in black and red colors) and the reported 5-principal elemental CoCrNi-based 3d TM HEAs at RT.^22,25–28)

Based on the phase analysis, the as-cast alloy is composed of a single BCC solid solution phase. According to Peierls-Nabarro Model $\tau _{\textit{PN}} \propto 2Ge^{ - \frac{2\pi W}{b}}/(1 - \nu )$ (where W is the dislocation width, G is the shear modulus, ν is the Poisson’s ratio and b is the modulus of Burgers vector), the Perierls stresses of BCC and FCC are closely related with the value of W/b. The value of W/b in BCC metals is commonly smaller than that in FCC metals in the cubic crystals,²⁹⁾ which indicates that the Peierls stresses in BCC metals are likely to be larger than those in FCC metals. Hence, the dislocation movement in BCC metals is harder than that in FCC metals, resulting in a relatively high yield stress in BCC metals. Moreover, the as-cast alloy was prepared by rapidly solidifying the mixed principal elements into a cold copper hearth, which inevitably results in an extensive solid solution. Thus, the single BCC phase combined with the strong solid solution strengthening might take the responsibility for the high ultimate strength and extremely high hardness. After annealing, the (Ni, Co)₃Ti phase and the FCC phase form, relaxing the solid solution strengthening, which facilitates the hardness decreasing. On the contrast, the diversification of phase component due to the above two phases precipitated directly results in a great increase of phase interface, further enhancing the strength of the annealed alloy, which is consistent with that the precipitation of strong and brittle laves phase dramatically increases the yield strength of 230 MPa of CoCrCuFeNi to 1272 MPa of the CoCrCuFeNiTi alloy.¹⁴⁾

In summary, a typical single phase solid solution was identified in the as-cast CrCoNiTiV alloy with an outstanding hardness around 825.8 ± 16.3 HV. Its micromorphology was divided into the CrV-rich dendrites, CoTi-rich interdendrites and Ni-rich regions. After annealing, multi-phases belonging to two BCC phases (BCC1+BCC2), an FCC phase and a (Ni, Co)₃Ti phase, form relaxing the extent of solid solution of the as-cast alloy, which results in the decreasing harness to 680 ± 23.9 HV. In contrast, the ultimate strength significantly increases from 1729 ± 15 MPa of the as-cast alloy to 2820 ± 15 MPa of the annealed alloy, which might originate from the diversification of phase component (the majority BCC phase + other precipitation).

Financial supports from Natural Science Foundation of Jiangsu Province of China (Grants No. BK20181047) and Jiangsu Province of China Innovation and Entrepreneurship Program “Doctor of Innovation and Entrepreneurship” (Grants No. KYY18017) are gratefully acknowledged.

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